A wash coat method has been known as a method of coating a catalyst slurry on the inner wall of the honeycomb channels formed to a monolith as a catalyst substrate.    Patent Document 1: JP-A No. 63-51949    Patent Document 2: JP-W 2002-506720
FIG. 5 shows a coating device by the existent wash coat method, in which a slurry S is supplied from a slurry supplying device 53 to a storing part 52 formed on the side of the upper end of a monolith M disposed such that honeycomb channels 51 - - - are opened at upper and lower ends, and the slurry is caused to flow into and spread in the flow channels 51 by supplying a pressurized air on the side of the upper end or sucking air on the side of the lower end to coat the inner wall of the channels 51.
Recently, a noble metal such as platinum is sometimes contained in the slurry S and the slurry S by a necessary amount used for coating the monolith M to the storing part 52.
By the way, since some slurry S has a high viscosity, the slurry supply nozzle 54 is necessary to have a diameter to some extent but it is desirably as small as possible for strictly controlling the supply amount. Then, when a slurry at a high viscosity is supplied from the supply nozzle 54 to the storing part 52, the slurry S is stored in a state raised at a portion just beneath the supply nozzle 54 (refer to FIG. 6(a)).
When a pressurized air is supplied from the upper end or air is sucked from the lower end in this state, the slurry S initially flows inward uniformly in each of the channels 51 - - - (refer to FIG. 6(b)).
Since the amount of the slurry S is large in the central portion and small in the peripheral portion, even after the slurry S at the peripheral portion has been caused to flow downward completely into the honeycomb channels 51A at the peripheral portion, the slurry still flows downward in the honeycomb channels 51B at the central portion.
Then, when the slurry is finally spread and coated in the honeycomb channels 51 by sucking air on the side of the lower end, the coating length for the honeycomb channels 51B in the central part is longer than the coating length in the honeycomb channels 51A at the peripheral part (refer to FIG. 6(c)).
Accordingly, in a case, for example, of supplying a slurry in a required amount intending to coat the slurry S by one-half length of the honeycomb channels 51, the honeycomb channels 51A at the peripheral part are coated only by the one-half length or less, whereas the honeycomb channels 51B at the central part are coated by one-half length or more to make the coating length not uniform.
In the same manner, in a case of supplying the slurry S by a necessary amount used for coating the monolith M, since a sufficient amount of the slurry S does not flow downward in the honeycomb channels 51A at the peripheral part, the honeycomb channels 51A are not coated as far as the lower end, whereas excess slurry S flows out from the honeycomb channels 51B at the central part (refer to Fig. (d)).
Accordingly, in order to coat the slurry S uniformly over the entire length for each of the honeycomb channels 51, it has to be used inevitably a method of supplying the slurry S in a much more amount than the necessary amount used for coating the monolith M to the storing part 52, filling the slurry S excessively in all the honeycomb channels 51 and cleaning off the surplus slurry S. Then, the cleaned off surplus slurry S has to be recovered, re-conditioned for the ingredient and recycled, and this results in troubles by so much.